問61
頻出度★☆☆
ペイント系ソフトウェアで用いられ，グラフィックスをピクセルと呼ばれる点の集まりとして扱う方法であるラスタグラフィックスの説明のうち，適切なものはどれか。
平成25年度 秋期 ITパスポート試験 問61
テクノロジ系情報メディア › マルチメディア技術
This question's explanation and your answers so far.
解説
問 61
Incorrectア
What is widely used in CAD (computer-aided design) is not raster graphics but vector graphics. CAD requires that the dimensions of a drawing be kept exact, that the position and length of lines can be edited numerically, and that lines do not become blurred even when part of the drawing is scaled up or down. Because the vector format stores shapes as coordinates and equations, it handles changes of dimensions and display at any magnification well, and keeps the precision required of a technical drawing. The raster format is nothing more than a collection of a fixed number of pixels: it becomes coarse when enlarged, and lines cannot be redrawn afterwards by specifying numeric values, so it is not suited to design work. This choice is therefore wrong.
- イ
正解 The correct answer is イ. Raster graphics (the bitmap format) divides an image into a grid of rows and columns and represents the whole image as a collection of dots, taking one cell of that grid, a pixel, as the smallest unit. The actual data is a list of “which pixel at which position has which color”, and every single pixel independently holds its own color information (the type of color, the brightness, and transparency if needed). For example, with 24-bit full color, each pixel holds an 8-bit value for each of red, green and blue, so even neighboring pixels can be given completely different colors or brightness. Thanks to this structure, images such as photographs, in which color and shading change subtly, can be reproduced faithfully, and in paint software you can edit at the pixel level, for instance repainting only a specific dot or correcting the brightness of just one part. Therefore the statement in イ, that the type of color and brightness can be adjusted pixel by pixel, is an appropriate description of raster graphics. By contrast, vector graphics stores an image as shape-definition data such as the coordinates of points, the equations of lines and curves, and fill attributes, so it has no concept of a pixel at all and no notion of specifying color pixel by pixel.
Incorrectウ
The data size of a raster image is determined roughly by (number of pixels horizontally × number of pixels vertically × number of bits of color information per pixel). In other words, if you raise the resolution and increase the number of pixels, the number of pixels that must be recorded increases accordingly, so the file size grows as well; it can never stay constant regardless of whether the resolution is high or low. Even for an image of the same size, full color and monochrome use a different number of bits per pixel, so the size differs. A data size that hardly changes when the resolution or the display magnification changes is a characteristic of vector graphics, which hold only the equations defining the shapes, so this is wrong as a description of raster graphics.
Incorrectエ
Raster graphics builds an image out of a finite number of pixels, so when it is enlarged each pixel is stretched into a square block, and stair-like jagged edges (jaggies) become conspicuous along the edges of diagonal lines and curves. Anti-aliasing can place intermediate colors to make them less noticeable, but that does not mean jaggies do not occur in principle. Keeping outlines smooth even when enlarged is a strength of vector graphics, which can redraw the shapes from equations every time they are displayed. This is therefore wrong as a description of raster graphics.
ここがポイント
- Raster (bitmap) graphics is a method that stores an image as a collection of pixels, with each pixel holding information about color and brightness. It suits images with fine gradations, such as photographs, and is handled by paint software. Its data size is proportional to the number of pixels × the number of bits per pixel, so raising the resolution increases the file size, and enlarging the image produces jaggies.
- Vector graphics is a method that stores shapes as coordinates and equations; outlines stay smooth when scaled up or down, the data size depends little on resolution, and it is used in CAD and draw software. A standard exam point is to memorize the strengths and weaknesses of the two as a pair (pixel-level editing versus tolerance of enlargement, size characteristics, and typical uses).
用語